Method of removing a thermal barrier coating

ABSTRACT

A method of removing a ceramic coating, such as a thermal barrier coating (TBC) of yttria-stabilized zirconia (YSZ), from the surface of a component, such as a gas turbine engine component. The method generally entails subjecting the ceramic coating to an aqueous solution of ammonium bifluoride, optionally containing a wetting agent, such as by immersing the component in the solution while maintained at an elevated temperature. Using the method of the invention, a ceramic coating can be completely removed from the component and any cooling holes, with essentially no degradation of the bond coat.

FIELD OF THE INVENTION

[0001] The present invention relates to methods for removing ceramiccoatings. More particularly, this invention is directed to a method forremoving a layer of thermal-insulating ceramic material, such asyttria-stabilized zirconia (YSZ) from the surface of a componentintended for service at high temperatures, such as a component of a gasturbine engine.

BACKGROUND OF THE INVENTION

[0002] Components located in certain sections of gas turbine engines,such as the turbine, combustor and augmentor, are often thermallyinsulated with a ceramic layer in order to reduce their servicetemperatures, which allows the engine to operate more efficiently athigher temperatures. These coatings, often referred to as thermalbarrier coatings (TBC), must have low thermal conductivity, stronglyadhere to the article, and remain adherent throughout many heating andcooling cycles.

[0003] Coating systems capable of satisfying the above requirementstypically include a metallic bond coat that adheres thethermal-insulating ceramic layer to the component. Metal oxides, such aszirconia (ZrO₂) partially or fully stabilized by yttria (Y₂O₃), magnesia(MgO) or other oxides, have been widely employed as the material for thethermal-insulating ceramic layer. The ceramic layer is typicallydeposited by air plasma spraying (APS), low pressure plasma spraying(LPPS), or a physical vapor deposition (PVD) technique such as electronbeam physical vapor deposition (EBPVD), which yields a strain-tolerantcolumnar grain structure. Bond coats are typically formed of anoxidation-resistant diffusion coating such as a diffusion aluminide orplatinum aluminide, or an oxidation-resistant alloy such as MCrAlY(where M is iron, cobalt and/or nickel). Aluminide coatings aredistinguished from MCrAlY coatings, in that the former are primarilyaluminide intermetallic while the latter are a metallic solid solutionthat contains a mixture of phases, including βNiAl.

[0004] Though significant advances have been made with coating materialsand processes for producing both the environmentally-resistant bond coatand the thermal-insulating ceramic layer, there is the inevitablerequirement to remove and replace the ceramic layer under certaincircumstances. For example, removal may be necessitated by erosion orimpact damage to the ceramic layer during engine operation, or by arequirement to repair certain features such as the tip length of aturbine blade. Removal of the ceramic layer may also be necessitatedduring component manufacturing to address such problems as defects inthe coating, handling damage and the need to repeat noncoating-relatedmanufacturing operations which require removal of the ceramic, e.g.,electrical-discharge machining (EDM) operations.

[0005] Current state-of-the-art repair methods often result in removalof the entire TBC system, i.e., both the ceramic layer and bond coat,after which the bond coat and ceramic layer must be redeposited. Onesuch method is to use abrasives in procedures such as grit blasting,vapor honing and glass bead peening, each of which is a slow,labor-intensive process that erodes the ceramic layer and bond coat, aswell as the substrate surface beneath the coating. With repetitive use,these procedures eventually destroy the component by reducing the wallthickness of the component. This disadvantage is particular acute withdiffusion aluminide bond coats, which have a diffusion zone that extendsinto the substrate surface of the component. Damage to diffusionaluminide bond coats generally occurs by the fracturing of brittlephases in the diffusion zone, such as PtAl₂ phases of aplatinum-aluminide bond coat, or in the additive layer, which is theoutermost bond coat layer containing an environmentally-resistantintermetallic phase MAl, where M is iron, nickel or cobalt, depending onthe substrate material. Damage is particularly likely when treating anair-cooled component, such as a turbine blade whose airfoil surfacesinclude cooling holes from which cooling air is discharged to cool theexternal surfaces of the blade.

[0006] Consequently, significant effort has been directed to developingnonabrasive processes for removing ceramic coatings. One such method isan autoclaving process in which the ceramic coating is subjected toelevated temperatures and pressures in the presence of a causticcompound. This process has been found to sufficiently weaken thechemical bond between the ceramic and bond coat oxide layers to permitremoval of the ceramic layer while leaving the bond layer intact.However, suitable autoclaving equipment is expensive, and autoclavingtechniques have been incapable of removing ceramic from the coolingholes of an air-cooled turbine blade.

[0007] Accordingly, what is needed is a process capable of removing aceramic layer from a component without damaging an underlying substrate,including any bond coat used to adhere the ceramic layer.

SUMMARY OF THE INVENTION

[0008] The present invention provides a method of removing a ceramiccoating, such as a thermal barrier coating (TBC) of yttria-stabilizedzirconia (YSZ), from the surface of a component. Particularly notableexamples are gas turbine engine components exposed to the hostilethermal environment of the turbine, combustor and augmentor sections ofa gas turbine engine. The method is particularly suited for completelyremoving a thermal-insulating ceramic coating of a thermal barriercoating system without removing a metallic bond coat, such as adiffusion aluminide or MCrAlY coating, that adheres the ceramic coatingto the surface of the component.

[0009] The method of this invention generally entails subjecting theceramic coating to an aqueous solution of ammonium bifluoride. Apreferred process for removing the ceramic coating further entailsimmersing the component in the solution while maintained at an elevatedtemperature, and subjecting the coating to ultrasonic energy. Using themethod of this invention, a ceramic coating can be completely removedfrom the component and any cooling holes, with essentially nodegradation of the bond coat. Accordingly, this invention allows thedeposition of a new ceramic coating on components in production withoutrefurbishment or replacement of the bond coat and without depositingadditional ceramic in the cooling holes, which would be detrimental tothe performance of the component. If the component has been in service,such that the bond coat has been partially depleted as a result ofoxidation, the bond coat can be refurbished before replacing the ceramiccoating.

[0010] A significant advantage of this invention is the reduced labor,equipment and processing costs required to remove a ceramic coating of athermal barrier coating system. In addition to the simplified processand equipment that can be used, labor and process costs are furtherreduced by avoiding damage and removal of the bond coat. In addition,the service life of a component can also be extended by avoidingreplacement of its entire thermal barrier coating system, since removalof a bond coat results in loss of wall thickness, particularly if thebond coat is a diffusion aluminide which inherently shares a significantdiffusion zone with the component substrate. Importantly, prior arttechniques for removing a ceramic layer of a TBC have typically beenunable to remove ceramic from cooling holes, or have caused excessivedamage to the bond coat in the process of removing the ceramic. Bycompletely removing ceramic from the cooling holes of an air-cooledcomponent, the performance of the component is improved by the restoreduniform film cooling of its surfaces.

[0011] Other objects and advantages of this invention will be betterappreciated from the following detailed description.

DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a sectional view of a surface portion of a gas turbineengine blade protected by a thermal barrier coating system that includesa ceramic layer adhered to the blade surface with a bond coat.

DETAILED DESCRIPTION OF THE INVENTION

[0013]FIG. 1 represents a partial cross-sectional view of an airfoilportion of a gas turbine engine turbine blade 10. The substrate 12 ofthe blade 10 is shown as being protected by a thermal barrier coatingsystem 14 composed of a ceramic layer 18 adhered to the substrate 12 bya bond coat 16. The method of this invention is directed to removing theceramic layer 18 from the substrate 12 of the blade 10 without removingor damaging the bond coat 16.

[0014] As is the situation with high temperature components of a gasturbine engine, the blade 10 may be formed of an iron, nickel orcobalt-base superalloy. The bond coat 16 is an oxidation-resistantcomposition such as a diffusion aluminide and/or MCrAlY, both of whichform an alumina (Al₂O₃) layer or scale (not shown) on its surface duringexposure to elevated temperatures. The alumina scale protects theunderlying superalloy substrate 12 from oxidation and provides a surfaceto which the ceramic layer 18 more tenaciously adheres. The ceramiclayer 18 can be deposited by air plasma spraying (APS), low pressureplasma spraying (LPPS) or a physical vapor deposition technique, e.g.,electron beam physical vapor deposition (EBPVD), which yields astrain-tolerant columnar grain structure (not shown). A preferredmaterial for the ceramic layer 18 is zirconia partially stabilized withyttria (yttria-stabilized zirconia, or YSZ), though zirconia fullystabilized with yttria could be used, as well as zirconia stabilized byother oxides, such as magnesia (MgO), calcia (CaO), ceria (CeO₂) orscandia (Sc₂O₃).

[0015] The method of this invention entails removing the ceramic layer18 without removing or damaging the bond coat 16, so that a new ceramiclayer can be deposited on the original bond coat 16. According to thisinvention, the ceramic layer 18 is preferentially removed by exposure toan aqueous stripping solution of ammonium bifluoride (NH₄HF₂) at anelevated temperature. A suitable composition for the aqueous solution isabout 20 to about 70 grams of ammonium bifluoride per liter of solution.To enhance the effectiveness of the solution, about 0.05 to about 0.2volume percent of a wetting agent can be added to the solution. Whilenot required, distilled or de-ionized water is preferred. Suitablewetting agents contain about 1 to 3 weight percent polyethylene glycolwith the balance being octylphenoxy polyethoxyethanol, with a preferredwetting agent being available under the name TRITON X-100 from UnionCarbide. A preferred composition for the stripping solution using theTRITON X-100 wetting agent is about 30 to about 40 grams of ammoniumbifluoride per liter of solution, about 0.1 to about 0.2 volume percentTRITON X-100, and the balance distilled or de-ionized water.

[0016] An optional ingredient for the solution is acetic acid (CH₃COOH).A suitable composition for a stripping solution containing acetic acidis about 10 to about 20 grams acetic acid per liter of solution, about20 to about 70 grams of ammonium bifluoride per liter of solution, about0.05 to about 0.2 volume percent wetting agent, with the balance beingessentially distilled or de-ionized water. Suitable stripping solutionshave been prepared and used that contain, per liter of solution, about16 grams of acetic acid and about 35 grams of ammonium bifluoride, about0.1 volume percent of the TRITON X-100 wetting agent, with the balancebeing water.

[0017] A suitable temperature range for the stripping process of thisinvention is about 140° F. to about 170° F. (about 60° C. to about 77°C.), more preferably about 140° F. to about 155° F. (about 60° C. toabout 68° C.). The stripping treatment of this invention also preferablyincludes the use of ultrasonic energy transmitted through the solutionto the ceramic layer 18. Frequencies of about 20 kHz to 40 kHz have beenfound suitable for ultrasonic energy levels of about 50 to about 200 Wper gallon (about four liters) of solution. The ultrasonic treatment canbe continued until the ceramic layer 18 is completely removed or atleast sufficiently loosened so that it can be removed by brushing orpressure spray rinsing, typically in about two to five hours. Withoutultrasonic treatment, a total treatment duration of about four to aboutfive hours is generally sufficient to weaken the chemical bond betweenthe ceramic layer 18 and the alumina scale on the bond coat 16. It isforeseeable that longer or shorter durations may be preferred, dependingon the properties of the particular coating system.

[0018] In practice, the stripping solution of this invention has beenused to remove YSZ TBC from nickelbase superalloy airfoils withoutdamaging an underlying platinum aluminide bond coat. Notably, TBC withinthe cooling holes of the airfoils was also removed, while portions ofthe bond coats within the holes and coated by the TBC, as well asuncoated bond coat within internal cooling passages of the airfoils,were not attacked. Following the treatment, the airfoils weresuccessfully recoated with TBC without requiring any refurbishing of thebond coat. Because the stripping process of this invention completelyremoved the TBC from the cooling holes, the thermal performance of theairfoils was not impaired by the accumulation of excess TBC in thecooling holes. Furthermore, because the stripping process of thisinvention did not damage any portions of the bond coats, additionalprocessing steps to repair or replace the bond coats were unnecessary.If the blades had been in service, such that the bond coats werepartially depleted as a result of oxidation, the bond coats, whether adiffusion aluminide or MCrAlY-type, could be refurbished prior to TBCdeposition using a diffusion aluminizing technique such as packcementation or vapor phase aluminizing.

[0019] While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

What is claimed is:
 1. A method of removing at least a portion of aceramic coating from a component, the method comprising the step ofsubjecting the portion of the ceramic coating to an aqueous solutioncomprising ammonium bifluoride.
 2. A method according to claim 1 ,wherein the subjecting step comprises immersing the component in thesolution.
 3. A method according to claim 2 , wherein the subjecting stepfurther comprises directing ultrasonic energy at the ceramic coatingwhile the component is immersed in the solution.
 4. A method accordingto claim 1 , wherein the solution comprises about 20 to about 70 gramsof ammonium bifluoride per liter of the solution, the balanceessentially water.
 5. A method according to claim 1 , wherein thesolution comprises about 20 to about 70 grams of ammonium bifluoride perliter of the solution, about 0.05 to about 0.2 volume percent wettingagent, the balance essentially water.
 6. A method according to claim 1 ,wherein the solution further comprises acetic acid.
 7. A methodaccording to claim 6 , wherein the solution comprises about 10 to about20 grams of acetic acid per liter of the solution, about 20 to about 70grams of ammonium bifluoride per liter of the solution, about 0.05 toabout 0.2 volume percent wetting agent, the balance essentially water.8. A method according to claim 1 , wherein the wetting agent comprisesabout 1 to 3 weight percent polyethylene glycol with the balance beingessentially octylphenoxy polyethoxyethanol.
 9. A method according toclaim 1 , wherein the subjecting step further comprises heating thecomponent and the solution to about 60° C. to about 68° C.
 10. A methodaccording to claim 1 , wherein the component further comprises ametallic bond coat adhering the ceramic coating to the component, andwherein the solution does not remove the bond coat.
 11. A methodaccording to claim 10 , wherein the bond coat is a diffusion aluminide.12. A method according to claim 1 , further comprising the step ofdepositing a ceramic material on a surface of the component exposed whenthe portion of the ceramic coating was removed.
 13. A method accordingto claim 1 , wherein the component is a component of a gas turbineengine.
 14. A method of removing a thermal-insulating YSZ layer from agas turbine engine component without removing a metallic bond coat on asurface of the component that adheres the YSZ layer to the component,the method comprising the step of immersing the component in an aqueoussolution comprising ammonium bifluoride and a wetting agent comprisingcontain about 1 to 3 weight percent polyethylene glycol with the balancebeing essentially octylphenoxy polyethoxyethanol.
 15. A method accordingto claim 14 , wherein the immersing step further comprises directingultrasonic energy at the YSZ layer while the component is immersed inthe solution.
 16. A method according to claim 14 , wherein the solutioncomprises about 30 to about 40 grams of ammonium bifluoride per liter ofthe solution, about 0.1 to about 0.2 volume percent wetting agent, thebalance essentially water.
 17. A method according to claim 14 , whereinthe solution further comprises about 10 to about 20 grams of acetic acidper liter of the solution.
 18. A method according to claim 14 , whereinthe immersing step further comprises heating the component and thesolution to about 60° C. to about 68° C.
 19. A method according to claim14 , wherein the bond coat is a diffusion aluminide.
 20. A methodaccording to claim 14 , further comprising the step of depositing a YSZmaterial on a surface of the bond coat exposed when the YSZ layer wasremoved.